What is a Controller?

A Controller is simply a class file that is named in a way that can be associated with a URI.

Consider this URI:

example.com/index.php/blog/

In the above example, CodeIgniter would attempt to find a controller named blog.php and load it.

When a controller's name matches the first segment of a URI, it will be loaded.

Let's try it: Hello World!

Let's create a simple controller so you can see it in action. Using your text editor, create a file called blog.php, and put the following code in it:

Then save the file to your application/controllers/ folder.

Now visit the your site using a URL similar to this:

example.com/index.php/blog/

If you did it right, you should see Hello World!.

Note: Class names must start with an uppercase letter. In other words, this is valid:

<?php
class Blog extends CI_Controller {

}
?>

This is not valid:

<?php
class blog extends CI_Controller {

}
?>

Also, always make sure your controller extends the parent controller class so that it can inherit all its functions.

Functions

In the above example the function name is index(). The "index" function is always loaded by default if the
second segment of the URI is empty. Another way to show your "Hello World" message would be this:

example.com/index.php/blog/index/

The second segment of the URI determines which function in the controller gets called.

Let's try it. Add a new function to your controller:

Now load the following URL to see the comment function:

example.com/index.php/blog/comments/

You should see your new message.

Passing URI Segments to your Functions

If your URI contains more then two segments they will be passed to your function as parameters.

For example, lets say you have a URI like this:

example.com/index.php/products/shoes/sandals/123

Your function will be passed URI segments 3 and 4 ("sandals" and "123"):

Important: If you are using the URI Routing feature, the segments
passed to your function will be the re-routed ones.

Defining a Default Controller

CodeIgniter can be told to load a default controller when a URI is not present,
as will be the case when only your site root URL is requested. To specify a default controller, open
your application/config/routes.php file and set this variable:

$route['default_controller'] = 'Blog';

Where Blog is the name of the controller class you want used. If you now load your main index.php file without
specifying any URI segments you'll see your Hello World message by default.

Remapping Function Calls

As noted above, the second segment of the URI typically determines which function in the controller gets called.
CodeIgniter permits you to override this behavior through the use of the _remap() function:

public function _remap()
{
// Some code here...
}

Important: If your controller contains a function named _remap(), it will always
get called regardless of what your URI contains. It overrides the normal behavior in which the URI determines which function is called,
allowing you to define your own function routing rules.

The overridden function call (typically the second segment of the URI) will be passed as a parameter to the _remap() function:

Any extra segments after the method name are passed into _remap() as an optional second parameter. This array can be used in combination with PHP's call_user_func_array to emulate CodeIgniter's default behavior.

Processing Output

CodeIgniter has an output class that takes care of sending your final rendered data to the web browser automatically. More information on this can be found in the
Views and Output class pages. In some cases, however, you might want to
post-process the finalized data in some way and send it to the browser yourself. CodeIgniter permits you to
add a function named _output() to your controller that will receive the finalized output data.

Important: If your controller contains a function named _output(), it will always
be called by the output class instead of echoing the finalized data directly. The first parameter of the function will contain the finalized output.

Here is an example:

public function _output($output)
{
echo $output;
}

Please note that your _output() function will receive the data in its finalized state. Benchmark and memory usage data will be rendered,
cache files written (if you have caching enabled), and headers will be sent (if you use that feature)
before it is handed off to the _output() function.

To have your controller's output cached properly, its _output() method can use:if ($this->output->cache_expiration > 0)
{
$this->output->_write_cache($output);
}
If you are using this feature the page execution timer and memory usage stats might not be perfectly accurate
since they will not take into acccount any further processing you do. For an alternate way to control output before any of the final processing is done, please see
the available methods in the Output Class.

Private Functions

In some cases you may want certain functions hidden from public access. To make a function private, simply add an
underscore as the name prefix and it will not be served via a URL request. For example, if you were to have a function like this:

private function _utility()
{
// some code
}

Trying to access it via the URL, like this, will not work:

example.com/index.php/blog/_utility/

Organizing Your Controllers into Sub-folders

If you are building a large application you might find it convenient to organize your controllers into sub-folders. CodeIgniter permits you to do this.

Note: When using this feature the first segment of your URI must specify the folder. For example, lets say you have a controller
located here:

application/controllers/products/shoes.php

To call the above controller your URI will look something like this:

example.com/index.php/products/shoes/show/123

Each of your sub-folders may contain a default controller which will be
called if the URL contains only the sub-folder. Simply name your default controller as specified in your
application/config/routes.php file

CodeIgniter also permits you to remap your URIs using its URI Routing feature.

Class Constructors

If you intend to use a constructor in any of your Controllers, you MUST place the following line of code in it:

parent::__construct();

The reason this line is necessary is because your local constructor will be overriding the one in the parent controller class so we need to manually call it.

Constructors are useful if you need to set some default values, or run a default process when your class is instantiated.
Constructors can't return a value, but they can do some default work.

Reserved Function Names

Since your controller classes will extend the main application controller you
must be careful not to name your functions identically to the ones used by that class, otherwise your local functions
will override them. See Reserved Names for a full list.